1. Field of the Invention
This invention relates to an LC-type dielectric filter utilized in microwave band communication and more particularly to an LC-type dielectric filter using strip lines for resonators.
2. Brief Description of the Related Art
Recently, high frequency microwave band communications have had a great role in mobile communication systems, for example, in the recently developed cellular telephone systems. In this technology, since communications systems require several hundreds of frequency channels in the approximately 800 MHz frequency band, there has long been a need for a small filter, having a high quality factor or high-Q, and less parasitic capacity, and which is suitable for mass-production.
One example of a conventional filter is disclosed in an article entitled "Dielectric Filter having Attenuation Pole for Microwave Band", OKI ELECTRIC INDUSTRY CO., Research & Development, No 144, Vol. 56, No. 1 published on Jan. 1, 1989.
FIG. 1 illustrates a four resonator type uni-block dielectric filter disclosed in the above mentioned article. As shown in FIG. 1, the filter comprises a single rectangular dielectric block D.sub.1. The dielectric block D.sub.1 has four cylindrical holes H.sub.1 to H.sub.4 having metalized interior surfaces and metalized portions M.sub.1 to M.sub.10 on the block surfaces, with the metalized portions M.sub.2, M.sub.4, M.sub.6 and M.sub.8 connected to the metalized interior surfaces.
In this configuration of FIG. 1, each of the holes performs as a short-circuited 1/4 wavelength coaxial resonator. The respective spaces between the metalized portions M.sub.3, M.sub.5, and M.sub.7, and the metalized portions M.sub.2, M.sub.4, and M.sub.6 perform the function of coupling capacitances between the resonators.
FIG. 2(a) and FIG. 2(b) illustrate another example of a conventional dielectric filter, which is disclosed in Japanese Kokai publication No. 62-265658 published on Nov. 18, 1987, wherein FIG. 2(a) illustrates a front side of the filter and FIG. 2(b) illustrates a reverse side of the filter.
As shown in FIG. 2(a), a main body of the filter comprises a dielectric plate D.sub.2 having four through holes H.sub.5 to H.sub.8. Further, on the front side of the dielectric plate D.sub.2, there are provided three spiral printed coils L.sub.1A, L.sub.2A, and L.sub.3A for inductance of the filter and three metalized portions C.sub.1A, C.sub.2A, and C.sub.3A for capacitance of the filter. Each of the inductances and capacitances is electrically combined with a corresponding similar configuration provided on the reverse side of the dielectric plate D.sub.2.
As shown in FIG. 2(b) on the reverse side of the dielectric plate D.sub.2, there are provided four metalized portions C.sub.1B, C.sub.2B-1, C.sub.2B-2, and C.sub.3B which are coupled with the above mentioned metalized portions C.sub.1A, C.sub.2A, and C.sub.3A via the dielectric material of the dielectric plate D.sub.2 for forming capacitors of the filter. Further, there are provided three printed coils L.sub.1B, L.sub.2B, and L.sub.3B for forming inductors of the filter. According to this configuration, because the diameters of the coils on each side are different, the parasitic capacitance between the coils can be reduced and the frequency characteristic of the filter can be improved, as is described in detail in the Japanese Kokai Publication.
However, the above-mentioned conventional dielectric filters have certain disadvantages.
As to the first example shown in FIG. 1, it is very difficult to make a cylindrical hole in the dielectric block with sufficient accuracy because the dielectric material is very hard. Especially, when an adjustment of the filter is to be made, it is necessary to scrape the dielectric material which, in many cases, consists of very hard ceramics. Such a material is difficult to scrape even with a carbon silicon scraper. Further, it is also difficult to metalize the inner surfaces of the holes by plating. Therefore, this dielectric filter is not suitable for large scale production.
As to the second example shown in FIGS. 2(a) and 2(b), even though this type of filter is easy to make because conventional methods of manufacturing printed circuit boards may be applied, there is a fundamental problem: an amount of parasitic impedance will always be present because in a filter featuring one or more spiral coils each coil itself has parasitic impedance, such as stray capacitance between its electrodes.
Therefore, in fact, the quality factor of this kind of filter when not loaded may be up to approximately 100. This is why the filter is applicable for use only under the approximately 500 MHz frequency band. If the frequency exceeds 500 MHz, the parasitic impedance increases at an approximately exponential rate and it cannot satisfy the necessary frequency characteristic.